The present invention relates to a color CRT, and in particular improvement in the picture quality, specifically the resolution, of a color CRT.
Conventional color CRTs adopt various measures to improve the picture quality. However, new problems are encountered with increasing requirements regarding picture quality, particularly, resolution. Such problems are discussed below.
FIG. 25 shows a cross section of electrodes in a conventional electron gun shown for instance in an article, S. Shirai, et al. "Enhanced Elliptical Aperture Lens Gun for Color Picture Tubes," Proceedings of SID, Vol. 31/3, 1990. In the figure, reference numeral 31 denotes an electrode to which an anode voltage is applied. Reference numeral 32 denotes a DBF (dynamic beam forming) electrode to which a voltage which varies depending on the position on the screen at which the electron beam is impinging. Reference numeral 33 denotes a focus electrode to which a constant voltage is applied. Reference numerals 34, 35, 36, and 37 denote a triode section and a pre-lens system for generating an electron beam. Reference numeral 37 denotes a G1 electrode, and 36 denotes a G2 electrode.
FIG. 26A and FIG. 26B show the details of two different examples the DBF electrode. In these figures, reference numeral 100 denotes an electron beam.
The operation will next be described.
The function of a deflection yoke, which is not illustrated but which is one of the constituting elements, will first be described, before describing the function of the electron gun.
The deflection yoke in the color CRT has the function of deflecting the three electron beams 100 to the respective point on the screen, but also the function of converging the three electron beams to a single point (self convergence function). This is necessary to improve the color purity.
First, let us consider a situation where the beams 100 are directed to the center of the screen. In this case, no magnetic field is generated by the deflection yoke. The lens of the electron gun is designed so that the electron beams 100 are emitted at an angle with respect to each other so that the three beams converge at a point on the screen.
The situation when the electron beams which are emitted at an angle with respect to each other are deflected by the deflection yoke will next be described.
When the beams are deflected by the deflection yoke, the length of the trajectory of the beam to the screen will be longer than when the beam is directed to the center. If the beams are simply deflected, the center beam (G beam) and side beams (R and B beams) cross each other before the screen, so that they are not converged on the screen. This means that the desired color is not reproduced at the desired position.
To solve this problem, the deflection yoke generates a magnetic field for horizontal deflection which is increased with the distance from the center axis. The magnetic field having such a distribution is called a pincushion magnetic field. The magnetic field having the opposite distribution is called a barrel magnetic field.
When the magnetic field is generated, the beam on the outer side is subject to a greater deflection, while the beam on the inner side is subject to a smaller deflection. As a result, the beams tend not to cross before the screen.
By optimizing the functions of the magnetic field, the three beams can be made to converge on the screen.
If this function is seen as an action of a lens, it can be considered as a diverging lens, because there is a function of increasing the distance of the side beams in the horizontal direction. In the vertical direction, the magnetic field forms a converging lens by nature.
However, selecting the pincushion magnetic field alone is not enough to converge the three beams. This is because the freedom in adjustment is limited by restrictions. For instance, even if the R and B beams may be made to converge, the G beam may be offset from the point where the R and B beams are converged. For this reason, generally, a barrel magnetic field which is opposite to the pincushion magnetic field is generated at the neck part of the deflection yoke. The barrel magnetic field is a type of a 6-pole magnetic field, so that it acts on the G beam but imparts operations in an opposite direction to the R and B beams. By adjusting this magnetic field, in combination with the pincushion magnetic field, the three beams can be converged on the screen.
With regard to the horizontal direction, the three beams are converged throughout the screen because the lens for converging the beams is positioned between the electron gun and the screen. Accordingly, the focusing in the horizontal direction is satisfactory.
However, with regard to the vertical direction, because the converging lens is between the electron gun and the screen, the beams are in the state of overfocusing on the screen. For this reason, the picture on the screen is blurred.
In the conventional electron gun, the DBF electrode 32 shown in FIG. 25 forms a vertically diverging lens, so that it cancels the converging effect by the deflection yoke, to obtain a satisfactory focusing characteristic in the vertical direction on the screen.
To summarize the lens functions, a horizontally diverging lens is formed of the deflection yoke, while a vertically converging lens is formed of the deflection yoke. To prevent over-convergence in the vertical direction, the electron gun is provided with the DBF electrode 32 to which a voltage dependent on the position of the screen at which the beam is impinging is applied, so that the over-convergence in the vertical direction is alleviated.
According to the above prior art, a quadrupole (4-pole) electrode is generated in the electron gun to cope with the variation in the focusing characteristic generated by the deflection yoke. For this reason, a power supply for the generation of the quadrupole electric field is required, and the cost of the system is increased.
When seen as lenses, a horizontally diverging lens and a vertically converging lens are present near the screen, so that the magnification factor on the screen is different between horizontal and vertical directions.
Next, another prior art device will be described. FIG. 27A, FIG. 27B, FIG. 28A and FIG. 28B show a deflection system in a CRT disclosed in Japanese Patent Application Kohyou Publication No. 508,514/1993. FIG. 27A shows the general configuration of the system. FIG. 27B shows the astigma correction element 24 formed of a quadrupole electromagnet and a 45 degree-shifted quadrupole electromagnet. FIG. 28A shows a magnetic field generated by the quadrupole electromagnet, and FIG. 28B shows a waveform of a drive current for the quadrupole electromagnet.
This prior art device reduces the 6-pole magnetic field of the self-convergence yoke, to reduce the astigma generated by the 6-pole magnetic field, and supplies the two sets of quadrupole electromagnets with a dynamic drive current which varies depending on the beam spot, so that the three beams are converged.
The quadrupole magnetic field of the element 24 has the diverging function with respect to focusing, while the basic converging function is provided by the main lens of the electron gun. For this reason, the main plane of the converging lens system is made to approach the electron gun, and the magnification factor of the image is enlarged. As a result the spot diameter is increased. Moreover, it is necessary to supply the two sets of the quadrupole electromagnets with a dynamic drive current. The cost of the waveform generator and the power supply will then be considerable.